Conventionally, a variable valve timing apparatus which adjusts a relative angular phase between a crankshaft and a camshaft according to a braking torque generated by an actuator is known. The relative angular phase may be called as an engine phase indicating a valve operating timing. One of the variable valve timing apparatus is disclosed in JP 2008-51093A. The apparatus generates a braking torque by a fluidic actuator for adjusting the engine phase.
In detail, the apparatus in JP 2008-51093A has an actuator for generating a braking torque. The actuator is provided with a case, a rotor arranged in the case, a magneto-rheological fluid in contact with the rotor, and an electromagnetic device which adjusts a viscosity of the magneto-rheological fluid. The rotor has a shaft which penetrates the case and extends between an inside and outside of the case. The rotor is engaged with a phase adjusting mechanism. According to this apparatus, the rotor generates a braking torque according to the adjusted viscosity of the magneto-rheological fluid. The braking torque is transmitted to the phase adjusting mechanism. As a result, the phase adjusting mechanism adjusts the engine phase according to the braking force.
One technical problem of this kind of variable valve timing apparatus is that the magneto-rheological fluid which causes instability of various characteristics.
One technical problem of this kind of variable valve timing apparatus is a leak of the magneto-rheological fluid. A leak may change generating characteristics and input characteristics of the braking torque. In addition, a leak may affect adjusting characteristics of the engine phases and generates undesirable change on the characteristics.
One technical problem of this kind of variable valve timing apparatus is a pressure change in a fluid chamber where the magneto-rheological fluid is kept. Durability of the apparatus may be reduced if the pressure change excessively increased. In addition, the pressure change is not stable since the pressure change is generated according to an environmental temperature or a heat generation by a braking action.